From the outset, let me say that I do not have the absolute answer. But a few principles of study might help.
To answer such a question, first discover the context. ("A text without a context is a pretext".)
Look at its litererary and theological placement in the broad theme of the writing, not just the immediate verses but also the adjacent chapters and the overall structure of the writing.
More importantly, unearth the cultural context. When was it written, by whom, what was the purpose of that sector of the Church, and against which "heresy" were they aiming their material. Everything was written for a purpose; they were not creating nursery tales that were aimed simply to amuse. Where was it written, what were their idioms, understandings, and such.
The Acts of the Apostles was not the only "Acts" written at that time, so investigate and compare those. The "final" decision on the canon of Scripture was not made until centuries later, by the sector of the church that became dominant.
The Acts of the Apostles is recognised as historically unreliable, where the writer presented stories in the manner that it was wished should have happened. It was penned about the end of the first century, some 40 years after Paul's death, and at a time of great stress within the Movement.
Their leader had been put to death, the promised imminent coming had not taken place, and they were struggling to make sense of what was happening. The Jews only understood that the Messiah would be a fearful powerful warrior king, removing all foes from God's land and people. This Jesus/Joshua was nothing like all that the Scriptures said about the Messiah. The Jews knew nothing about the Messiah coming twice.
So with the Christians at the end of the first century becoming a laughing stock at focusing on this dead Messiah, the Christians responded by searching the Hebrew Scriptures to explain the reason for their predicament, and in the process silence and hopefully denigrate the Jews. Anti-Semitism finds its roots, and this results in many anti-Semitic sentiments being introduced into the Christian literature.
When they were able to find a few Hebrew texts that might have just the faintest relevance to Jesus, the Christians twisted the original meanings to make them appear relevant. Texts such as at Isa 7, Isa 53, and Ps 22 in particular were re-engineered to take them away from their original meaning so they could make them instead be applied to their dead leader.
So it is possible that it did not matter what part of Isaiah the Ethiopean was said to be reading; that would not have been the writers' concern or objective.
Consider, also that the use of an Ethiopean might have been relevant to the location where the Acts of the Apostles might have been written, Alexandria.
And there was a number of significant black early Church Fathers.
Doug